The watch can use LTE data for apps. However, you must contact your network to get a data plan for your watch, at a cost of course. The watch data plan will be linked to your phone contract. The data plan will be an eSIM and it can added to the watch using the Galaxy Wearable app. Do not forget to activate mobile data, like the phone if mobile data is deactivated it will not use it.
There is another thing. Making voice calls from your watch has different behaviour depending on your network. O2 does not allow watch to make voice calls from your watch if the the phone is turn off, plane mode or not connected to the network. Meaning that if the phone is in a place without signal, the watch cannot make phone calls even if it is in an area with good signal.
I've heard that EE does not impose such a restriction, but I cannot tell for sure. I do not know about other networks. O2 also stops you from downloading firmware.
Finally, to be able to make voice call you need to check the setting on the phone to allow phone calls from other devices. It is a bit tricky.
